A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device; which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A conventional lithographic apparatus includes a position control system or controller to control the position of the substrate support and other movable objects of the lithographic apparatus. This position control system includes a position measurement system configured to measure a position of the substrate support.
Important factors for the performance of a lithographic apparatus are, for example, the throughput, i.e. the number of wafers that is produced within a certain period, and the overlay, i.e. the production quality. In industry, there is a continuous demand to improve the throughput and overlay of the lithographic apparatus.
In the conventional lithographic apparatus, the substrate stage accuracy, which is measured in 6 degrees of freedom and is important for overlay, is controlled. Generally the two requirements of a higher throughput and a better overlay performance contradict each other, as higher accelerations used for higher throughput cause larger internal dynamic vibrations (or deformations) of the stages, which result in a deterioration of the substrate stage positioning accuracy.
Furthermore, disturbance forces may be exerted on the substrate stage during the lithographic process. These disturbance forces may also have a negative effect on positioning accuracy and/or settling time.
In view thereof, there is need for a position control system which is capable of controlling a stage with sufficient high speed and position accuracy.